Modern marine vessels that are for example employed in drilling operations often comprise a plurality of different electrical drives. These may for example be electric propulsion systems in which propellers are driven by electrical motors, such as Azimuth thrusters. Other types of electrical drives that require electric energy for operation include anchor winch drives, drives of drilling applications and the like. Electricity is generally produced on board and transmitted to the electric motor of the respective electrical drive.
For generating electric energy, marine vessels often comprise an on board power plant which produces the required electric energy by means of a diesel engine or a gas turbine coupled to a generator. Such arrangements provide the main power supply for the vessel. If the power plant experiences a blackout, or if there is a fault in the power transmission or distribution system, the vessel can loose its main power supply.
In case of such black out or another type of power loss, the electric drives will generally be disconnected and cease operation. For example in a hoisting application, such as the drawworks of a drilling rig or an anchor winch drive, brakes must be applied to the rotating motors and the operation will stop.
Furthermore, in such applications, energy may be generated by the respective electric motors during a hoisting operation, e.g. while lowering or stopping a load. The generated energy needs to be handled. This is generally performed by making use of water cooled braking resistors that burn off the energy. Drawwork motors, anchor winch motors, top drive motors or the like may for example have a drive setup with breaking choppers and resistors which burn off the braking energy generated by the motors. The problem of handling the braking energy is even more pronounced in system which comprise an active heave compensation, such as an active heave compensated drawworks on a drilling rig, in which braking energy is generated during the lowering operation for compensating the wave heights of the sea. The braking energy has to be handled in a more rapid fashion and is generally also burned off in braking resistors.
The water cooled braking resistors are generally heavy and require a substantial amount of space, while not having any further useful function. Furthermore, for example during active heave compensation, periodic peaks in the power consumption occur which is unfavorable as they result in a cyclic operation of the generators. Peaks in the energy consumption make the operation of the generators less efficient and furthermore require the power supply system of the marine vessel to be designed for larger loads. Furthermore, it is generally desirable to reduce the energy consumption of the marine vessel. Also, it is desirable to continue operation of the electrical drive for at least a short amount of time after a blackout of the power supply.